X-radiation is produced when moving electrons are decelerated or stopped due to collisions with the atoms of a substance. The intensity of this X-radiation is function of the accelerating voltage, the electron current, and the atomic number of the material bombarded.
In X-ray tubes, electrons from a hot cathode are focused into a small spot and accelerated to the anode or target. A television picture tube contains the same basic elements as an X-ray tube, that is a focused beam of electrons and a high D.C. accelerating voltage.
In computer monitor and color television picture tube applications, higher voltages are employed than in "black and white" applications, making X-ray emission absorption a very important consideration, particularly in the area of the glass tube face plate.
The prior art is replete with glass compositions directed to the absorption of X-rays, as for example U.S. Pat. Nos. 3,464,932, 4,015,966, 4,065,696, 4,065,697. Many of the glass compositions disclosed in the prior art may adequately absorb X-ray emissions but contain certain levels of materials that significantly interfere with the "standard" glass manufacturing processes.
The glass composition should be compatible with the refractories used in the glass melting and forming apparatus such that it does not cause the refractories to dissolve at an accelerated rate during glass melting and forming. In addition, the glass composition should not contain materials which will volatilize during glass melting and forming, since this will damage the refractory superstructures and also cause emissions of noxious odors and gases.
If the conventional molten tin float glass forming process is employed, as disclosed in U.S. Pat. Nos. 3,220,816 and 3,843,346, the glass composition should be compatible with the molten tin and should not contain any easily reducible oxides which will cause a film to form on the glass at the tin/glass interface. An easily reducible oxide will also tend to contaminate the tin bath with undesirable materials which may cause flaws in subsequently formed glass.
While certain of the prior art X-ray absorbing glass compositions have satisfied several of these important problems, none of these glass compositions have provided a glass which is particularly adapted to be manufactured by conventional float glass melting and forming processes and equipment and which demonstrates exceptional absorption of X-rays.